. There is increasing data to support the view that Pneumocystis carinii is not a single species but represents a diverse group of organisms related to the fungi. This collaboration has been initiated to further our understanding of the biodiversity of P. carinii. Work is in progress at the host institution (Rochester, New York, USA) to define the antigenic characteristics of P. carinii and to determine the extent of antigenic diversity among different isolates of P. carinii. Research at the foreign site (Oxford, UK) has been involved in the analysis of the genetic diversity of different isolates of P. carinii. Collaboration between the two groups will provide both antigenic and genetic data which together will provide a powerful tool for understanding of the complexity of the group of organisms encompassed by the term P. carinii. Ultimately, this will help in the understanding of the pathogenesis of P. carinii pneumonia and the prevention and treatment of the disease. The specific aims of this collaborative arrangement are to: 1. Study the genetic diversity among isolates of P. carinii from different mammalian host species. 2. Analyze genetic diversity among isolates of P. carinii from the same mammalian host. 3. Analyze genetic diversity among isolates of P. carinii within a single infected lung.